CaO-Al2O3-SiO2 Slag내 Ti 산화물의 열역학적 거동

Abstract

Titanium is added in many grade steels as an alloying element to improve mechanical properties via the grain refinement during hot rolling. Also, titanium is an important alloying element in ferritic stainless steels to stabilize C and N, and hence to improve the intergranular corrosion resistance of the weld. The titanium yield on its addition in liquid steel is very important to control the final Ti content in steel product. The stainless steelmaking process is consisted of various processes such as VOD (Vacuum Oxygen Decarburization) and LT (Ladle Treatment). There are different types of slags formed during these processes - CaO-Al2O3 type slag for VOD process and CaO-SiO2 type slag for LT process. In the present study, the Ti deoxidation experiments were carried out at 1873K using those slags to determine the titanium distribution ratios between slag and metal. The result showed that the titanium distribution ratio for CaO-Al2O3 type slag was about 10 while the ratio for CaO-SiO2 type slag was about 200. This implies that CaO-SiO2 type slag should not be used for high titanium yield in liquid steels. From the experimental results of titanium distribution ratios for various slag and melt compositions, the activity coefficient of TiOx, and the titanium oxide capacity, in slags were determined at 1873K using available thermodynamic data of Ti and O in liquid iron. The distribution ratio of Ti in CaO-SiO2 slag was much higher than the value in CaO-Al2O3 slag. Using thermodynamics of titanium and oxygen in liquid iron, the activity coefficients of TiO2, in those slags were determined. The co-deoxidation equilibria among Ti, Al or Si and O has been attained in liquid iron in the presence of slags containing respective oxides. The activity coefficient of TiO2 in CaO-SiO2 slag decreases as the slag basicity decreases, whereas it shows almost a constant value in CaO-Al2O3 slags indicating that the attraction between TiO2 and SiO2 is greater than that between TiO2 and Al2O3 in slag. In order to increase the yield of Ti in liquid steel during Ti addition practice, therefore, it is desirable to minimize the content of SiO2 in ladle slag.